(1) Field of the Invention
The present invention relates to an optical modulator, and in particular to an optical modulator applying identical modulation signals having a predetermined phase difference into a plurality of modulation electrodes.
(2) Related Art Statement
An optical modulation method for applying identical modulation signals having a predetermined phase difference (=time delay) into a plurality of modulation electrodes, such as an SSB (Single-Side band) modulation method, a duobinary modulation method or the like, has been conventionally utilized in the field of optical communications or optical measurement.
An SSB-type optical modulator consists of two sub Mach-Zehnder structures. In order to obtain SSB modulation signals, two splitted RF-signals with 90° phase difference by using an electrical 90° hybrid are applied to two sub-MZs modulation electrode respectively.
A dual-electrode Mach-Zehnder type optical modulator is used for the duobinary modulation method, whereby modulation signals respectively applied into the modulation electrodes of two branching waveguides are configured to have a predetermined bit difference therebetween by using a bit delay circuit. A bit delay is generally selected from the range from 0 to 2T (T=1 bit).
Because it is necessary to apply separate modulation signals respectively into the plurality of modulation electrodes in the SSB modulation method, the duobinary modulation method or the like as sated above, inputting a plurality of RF signals is required as the modulation signals for input. In addition, for the modulation methods giving a predetermined phase difference between each RF signal, an additional separate component, such as a delay circuit or the like, has been required to generate the phase difference. Therefore, there have been such problems as an increasing cost of the entire optical modulator, an increase of the setting space of delay circuit or the like. Furthermore, because phase difference accuracy of precisely 90° cannot be obtained by a commercially available 90° hybrid, it is necessary to separately use a phase adjuster for fine adjustment, and this makes the configuration more complicated.
Besides, in order to guide the plurality of modulation signals separately into the optical modulator, it is necessary to provide cables of different lengths between a signal source and interaction parts where the modulation signals act on lights in the optical waveguides, for generating a predetermined phase difference. Thus, the number of components is increased and the configuration gets complicated.
An object of the present invention is to solve the above described problems and provide a cost-saving and space-saving optical modulator enabling application of modulation signals with high phase difference accuracy without using the separate component for generating a phase difference outside of the optical modulator, when the identical modulation signals having a predetermined phase difference are applied into the plurality of modulation electrodes.